A growing number of analytical applications such as rapid screening of environmental samples, first response incident monitors and industrial scale process monitoring/control require sensitive real-time techniques which are accessible, rugged and relatively inexpensive.
It is well-documented that many byproducts of chlorine-mediated disinfection are putative or confirmed toxins and carcinogens; therefore, it is important to be able to monitor for their presence in drinking waters both during and after disinfection. Furthermore, the widespread use and storage of hydrocarbon fuels has resulted in numerous environmental releases into both ground and surface waters. Of particular concern, is the introduction of hydrocarbons into the environment via storm water run-off and leaking storage tanks. Rapid screening of a large number of samples will greatly facilitate the location of contaminated sites and point sources.
Currently, the accepted method for the analysis of volatile organic compounds (VOCs) and volatile disinfection byproducts (DBPs) in aqueous samples is purge-and-trap gas chromatography/mass spectrometry (P&T-GC/MS). This reliable analytical strategy requires a time-consuming chromatographic separation step, increasing the duty cycle analysis time (trap plus separation) and effectively reducing its utility as a real-time monitoring platform. As an alternative analytical technique, membrane introduction mass spectrometry (MIMS) was developed as a direct, real-time method for the analysis of volatile and semi-volatile organic compounds (VOC/SVOCs). Many drinking water disinfection byproducts (e.g. chloroform, dichloroacetonitrile) fall into this group and are easily and efficiently measured by MIMS. In this approach, sample is flowed over a semi-permeable membrane that provides on-line pre-concentration and permeation of analytes while excluding the bulk sample matrix. Analyte(s) are subsequently transferred (often by a carrier gas) to a mass spectrometer for detection. The online characteristics of MIMS (e.g. no sample preparation or chromatographic separation steps) make it ideal for the direct, real-time monitoring of analytes in complex samples. However, because mass spectrometry requires a vacuum system and relatively delicate components, it is inherently more fragile, therefore, is not amenable for widespread use in portable instrumentation. Additionally, the use of mass spectrometers requires a relatively high degree of technical proficiency in most cases, and although the cost of MS has dropped considerably in the past few years, it is generally considered too expensive for widespread use in municipal drinking water treatment facilities (e.g. online DBP monitoring) or storm water collection systems (e.g. VOC/SVOC contamination testing).
A number of membrane based techniques have been developed for direct analysis of samples and are reviewed recently by Jonsson and Mathiasson (J. Chromatogr. A, 902 (2000) p. 205-225). For example, membrane extraction with a sorbent interface (MESI) uses a hollow fiber membrane that is immersed in an aqueous sample. Volatile analytes permeate through the membrane and are stripped from the other side by a carrier gas, which is then flowed into a sorbent trap where analytes are integrated for thermal desorption onto a GC column. Detection is generally performed with a flame ionization detector (FID). Although MESI is a sensitive and selective methodology that can be programmed to provide chromatograms at regular, frequent intervals, it does not allow for continuous monitoring in real-time.
Thammakhet et. al. (J. Chromatogr. A, 1072(2) (2005) p. 243-248) employed an automated system with an adsorbent microtrap and thermal desorption inline with an FID to measure methane in gas samples, providing rapid data acquisition with no sample preparation. Purge-and-membrane with an electron capture detector (ECD), a similar technique, also avoids chromatographic separation. Instead, helium is bubbled through an aqueous sample, purging volatile and semivolatile analytes, which are subsequently collected by a hollow fiber membrane, followed by thermal desorption and detection. Although analytical quantification with purge-and-membrane ECD has been shown to be successful, it requires the complete purging of analytes from a sample of known volume, making it less suited for continuous duty real-time monitoring scenarios.
It is an object of the present technology to overcome the deficiencies of the prior art.